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Intrinsic foot muscles .The heart of balance ?

I recently placed a number of posts on a different part of the site under an Evolutionary heading but feel they are very relevant to the topic of footwear in as much as footwear might help or hinder the mechanisms discussed .

Here are the posts and I would welcome any comments .

Gerry

POST 1 The question I asked myself was what type of balance system can take the apparently difficult to balance , tall ,top heavy ,human phenotype and use this shape to advantage ?

Well , first consider balancing a pencil on your finger . Its very difficult .
Easier to balance is a long broom handle due to inertia . Easier still is the act of balancing a complete broom on your finger /palm with the broom head in the air and the end of the broom on your hand .
So can the head of the tibia balance in this way on the talus/calcaneal unit ? I think the answer may be yes , at least for postural stability in the medio lateral direction .

Previously , Luke Kelly (1) has shown that the intrinsic foot muscles can control foot posture including the condition of the medial longitudinal arch and ,in my opinion ,this could lead to inversion and eversion of the calcaneus, and then via the talus ,to a movement of the tibal head relative to the COG .

Another paper has shown that the vestibular apparatus has a direct link to some of the intrinsic foot muscles showing the have a key role in balance . (2)

A third paper (3) indicates that in the absence of strong intrinsics the extrinsics seem take on more of role in postural stability an so enlarge .

So what I am saying overall is that the intrinsics handle small medio lateral perturbations via calcaneal positioning and with larger perturbations the extrinsics kick in to assist .

This theory places the intrinsics right at the very heart of human balance .

https://www.researchgate.net/.../515...nsic_foot...22 Dec 2017 - Full-text (PDF) | The aim of this study was to determine the difference in activation patterns of the plantar intrinsic foot muscles during two quiet standing tasks with increasing postural difficulty. We hypothesised that activation of these muscles would increase with increasing postural demand...

Look at figure 6 in the paper below . If your not sure what I am talking about in the previous post this goes a long way to explaining things . ( figure 6 shows calcaneal inversion in direct response to electrical stimulation of the abductor halluces )

rsif.royalsocietypublishing.org/content/11/93/20131188
by LA Kelly - ‎2014 - ‎Cited by 50 - ‎Related articles
29 Jan 2014 - We test the hypotheses that activation of the three largest plantar intrinsic foot muscles, abductor hallucis, flexor digitorum and quadratus plantae is associated with muscle stretch in response to external load on the foot and that activation of these muscles (via electrical stimulation) will generate sufficient ...

Further to the above , an interesting paper was very recently published , Koyama et al 2017 (below) , which looked at fatiguing the muscles of the foot and the effect this has on postural sway . They found that the exercises reduced some aspects of postural sway .
The authors stated -

"This study revealed that fatiguing foot muscle exercises decreased foot muscle strength and altered postural sway during standing. Interestingly, the fatiguing foot muscle exercises decreased the COP range and velocity while standing compared with the pre-fatigue conditions. The decreased foot muscle strength after the exercises was not associated with changed postural sway during standing after the exercises ".

What makes this paper so interesting is that the exercises used in the study would have fatigued the extrinsic foot muscles but not the intrinsics ( calf raises and toe curls do not target the intrinsics and a toe grip dynamometer measures extrinsic toe flexor strength )

Contrary to the interpretation of the authors I believe this paper may show better postural stability with less extrinsic foot muscle input and greater reliance on the intrinsics .

journals.plos.org/plosone/article?id=10.1371/journal.pone.0189184
by K Koyama - ‎2017
7 Dec 2017 - The activities of the intrinsic and extrinsic plantar muscles contribute to postural stability during upright standing, especially in the single-leg standing [9]. Foot muscle strength is considered to be one of the important essentials that provides postural control while standing [10]; however, the relationship between foot muscle ...